Two main satellite-based navigation systems are operational today. The USA's Global Positioning System (GPS) and Russia's GLObal NAvigation Satellite System (GLONASS) GPS have been operational since 1978 and globally available since 1994. It consists of up to 32 medium earth orbit satellites in six different orbital planes, with the exact number of satellites varying as older satellites are replaced. GLONASS was a fully functional navigation constellation in 1995. After the collapsed Soviet Union it was recovered in 2011.
New systems like Galileo, COMPASS navigation system and IRNSS (Indian Regional Navigation Satellite System) is under development. IRNSS is an autonomous regional satellite navigation system under control of the Indian government. It will consist of a constellation of 7 satellites and is intended to map the India region. COMPASS is a Chinese system with up to 35 satellites in different orbits. It intends to be expanded to a global navigation system by 2020. Galileo is set up by the European Union and consists of 30 satellites providing global coverage. It will become operational in 2014 and reach full operation in 2020.
It is estimated that nine out of every ten new satellite navigation receivers currently sold are for civilian or commercial use. The current systems are thus widely used by civilians. The penetration rate is high for old Mobile phone, laptop, smart phone, or pad is equipped with a positioning device.
The common name for all these systems is Global Navigation Satellite Systems (GNSS). In these GNSS assistance solutions are needed and presently used to improve performance for Time To First Fix (TTFF), sensitivity and accuracy regarding positioning data. These assistance solutions could be of the following kinds:                Providing the positioning device with information regarding GNSS satellite positions for a given moment in the future, satellite clock off sets, PRN (Pseudo Random Noise) codes for the satellites etc. All together so called ephemeris and calendar data. This information is sent via a wireless network and hence the device must be equipped with a WWAN (Wireless Wide Area Network) or WLAN (Wireless Local Area Network) transceiver to be able to communicate with a mobile network and receive said data.        Providing a rough position by using information given by what base station in the mobile network the device is connected to in the case where the device is equipped with a WWAN transceiver. Hereby, together with ephemeris data provided or stored in memory in the device, the search space for the positioning device is decreased and TTFF, sensitivity and accuracy performance can be improved.        Providing a rough time to the positioning device provided via WWAN. It could be done with a running real time clock in the device or provided from external source. Hereby, together with ephemeris data provided or stored in memory in the device, the search space for the positioning device is decreased and TTFF, sensitivity and accuracy performance can be improved.        
If the GNSS equipped device does not have access to assistance solutions, or does not have valid broadcast ephemeris, almanac, position or timing data, the GNSS device must work in a so called autonomous mode. This means that it must search over frequency, time delay and PRN code space to find satellites and being able to compute the position.
GNSS devices are quite often used for navigation purposes. One user case could be that a person with a navigation device equipped UE (laptop, smart phone, pad or any other electronic device) where GNSS is not always on, turns on the navigation device when entering the car. The driver then needs to have a travel description before being able to start driving and choosing directions in the road system. TTFF and accuracy is critical in this case since the navigation device cannot give a travel description before position is known.
If the navigation device is equipped with an assistance solution and a WWAN or WLAN transceiver that is connected to a mobile network it will speed up TTFF. If not, the navigation device must work in the autonomous mode.
Both in assisted mode and in autonomous mode the TTFF could be too long for a positive user experience. Reasons could be weak satellite signal strength due to attenuation from vehicle body or no line of sight in urban canyons or indoor.